Signal indicating fuses differ from conventional fuses in that they comprise three, rather than two, terminals. The three terminals may be characterized as a common terminal, a normally closed (NC) terminal, and a normally open (NO) or alarm terminal. The portion of the fuse extending between the common and NC terminals corresponds to a conventional fuse. When functional it approximates a short circuit; after fuse failure it approximates an open circuit. The signal indicating fuse also includes a "flag" section extending between the common and the NO terminals. While the fuse remains functional, this section approximates an open circuit; after fuse failure a contact closure, approximating a short circuit, is made between the common and NO terminals.
The purpose of this invention is to provide a simple, economical apparatus for testing signal indicating fuses of various current ratings. This had been done by testing individual fuses with a power supply and a timer. Because this method is both time consuming and cumbersome, an improved procedure was sought. Furthermore, it is clearly more efficient to test a number, say N, fuses at a single test. However, given the apparatus then available, a test of N fuses required a power supply capable of providing a current on the order of N times the fuse rating.